A recent development in the food industry is the emphasis on intermediate-moisture foods which have the faculty of being stored and marketed in a substantially non-refrigerated condition. These foods were designed to avoid the need to be packaged in a hermetically sealed container and commercially sterilized or maintained in a frozen or refrigerated state throughout the period of distribution and storage by the consumer.
The intermediate-moisture foods are based on the principle of reducing the availability of the water in the food for microbial growth. The availability for spore germination and microbial growth is closely related to its relative vapor pressure, commonly designated as water activity. It was found that the use of a wide variety of water-soluble solutes, or osmotic agents, has the effect of depressing the water activity of the foods to levels at which most bacteria will not grow.
The water activity of a food is defined as the partial pressure of water in the food divided by the saturation pressure of water at the temperature of the food. The water activity can be determined by placing a sample in a container which is then sealed, and after equilibrium is reached, determining the relative humidity above the sample. Most products of this type have between 10 to 40% moisture, and a water activity between 0.65 and 0.9.
An early application of the technique of controlling water activity was for animal foods. For example, U.S. Pat. No. 3,202,514, issued Aug. 24, 1965, discloses an animal food having 15 to 30% moisture and 15 to 35% water-soluble solids, principally sugar, with a proteinaceous meaty substance. Subsequently, other foods were formulated with an intermediate-moisture content, such as egg products (U.S. Pat. No. 3,640,731, issued Feb. 8, 1972), pancake batter (U.S. Pat. No. 3,753,734, issued Aug. 21, 1973) and whippable bases for confectionary use (U.S. Pat. No. 3,958,033, issued May 18, 1976). The water content and water activity of these foods are brought to as low a value as practical to insure their long-term stability without refrigeration. The main difficulty with these foods is that their low-moisture content may detract from their palatability, texture and mouth-feed. This technique therefore has found its greatest commercial applicability in the pet-food market where palatability requirements are not as stringent.
It is of course desirable to be able to avoid refrigeration and freezing of food products to reduce the cost involved, and particularly for the consumer, to avoid the inconvenience of unpacking, handling, and then defrosting the typically rock-hard frozen foods. However, freezing is an extremely safe and suitable technique for long-term storage and provides the manufacturer with great leeway in incorporating any of a wide variety of ingredients in foods which would otherwise be short-lived.
It is an object of the present invention to provide a class of foods which are normally maintained at freezer temperature, but which possess the requisite microbiological stability for storage at refrigerator or room temperatures for an extended length of time without spoilage. Other objects, and advantages, of this invention will be apparent from the ensuing specification and examples.